The utility of stem cells (SCs), including hematopoietic SCs, mesenchymal SCs or multipotent adult progenitor cells such as endothelial progenitor cells (EPCs) and embryonic stem cells (ESCs), is well established, especially for generating multiple distinct cell types in medicine and/or veterinary applications and/or for animal improvement.
In particular, stem cells may be used as a source of cells that can be differentiated into various cell types to repopulate damaged cells. For example, joint pain is a major cause of disability, which most often results from damage to the articular cartilage by trauma or degenerative joint diseases such as primary osteoarthritis. Current methods of treatment for cartilage damage are often not successful in regenerating cartilage tissue to a fully functional state, and there is often considerable donor-site rejection. A resolution of this disease state can be provided by regenerating cartilage tissue using stem cells. There are many other tissue degenerative diseases, which can be treated using stem cells, including autoimmune disorders. For example, in the treatment and/or therapy of diabetes, the pancreatic islet cells of a diabetic patient can be regenerated using stem cells that are implanted and/or infused into the patient.
Despite the pluripotency of embryonic stem (ES) cells, legal and moral controversies concerning their use, and the lack of available human ES lines, have prompted researchers to turn to investigating new sources for isolating stem cells from tissues that are not of fetal origin. However, such adult stem cells still involve complicated isolation procedures, and are in limited supply.
Because of the numerous obstacles and technical difficulties in producing and using ES cells and adult stem cells in sufficient quantity for a large number of clinical applications, many researchers are now looking to develop strategies to reprogram somatic cells from adult tissues to thereby create cells having stem cell-like attributes, in particular the ability to differentiate into different cell types.
In one approach, mature cells are fused with embryonic germ cells by a process known as somatic-cell nuclear transfer (SCNT). After fusion, mature cell nuclei display pluripotent properties similar to that of the embryonic germ cells (Tada et al., 1997, EMBO J. 16:6510-6520). This fusion-process essentially returns the mature adult cell to an earlier developmental state (immature state), from which the cell can then mature into differentiated cell types. However, such reprogramming does not escape the requirement for isolated ES-cells or embryonic germ cells. Moreover, the ethical and religious issues associated with using human embryos apply equally to this technology. There are also practical difficulties in SCNT, including the short supply of human oocytes for SCNT.
In another approach, the sequential exposure of primary oligodendrocyte precursor cells (OPCs) to fetal calf serum and basic fibroblast growth factor (bFGF) produces cells that resemble multipotent stem cells (Kondo et al., 2000, Science 289:1754-1757). However, the procedure has not been shown to be applicable to other cells types and, as OPCs are not an abundant cell type, there is limited prospect for the large-scale application of this technology.
Finally, human fibroblasts have been shown to be capable of being made into pluripotent cells by ectopic expression of four factors: Oct3/4, Sox2, Klf4, and c-Myc (Kzutoshi et al., Cell 131:861-872 (2007); Park et al., Nature epub (2007)). The so-called “induced pluripotent stem cells” (iPSCs) produced by this technology were shown to be similar to human embryonic stem (ES) cells in morphology, proliferation, surface antigens, gene expression, epigenetic status of pluripotent cell-specific genes, and telomerase activity. On the other hand, the iPSCs were also shown to give rise to teratomas, raising concerns about the application of the technology to medicine and/or the veterinary industry and/or for animal improvement.
Accordingly, there is a need in the art for an abundant source of cells that are capable of being differentiated into different cell types without extracting or using egg cells or stem cells such as ES cells or the like, and with minimal deleterious effects. More particularly, there is a need in the art for alternative and/or improved methods of culturing differentiated cells and culture media suitable for producing cells capable of differentiating into a plurality of cells types, and which are preferably capable of supporting self-renewal of cells having this capacity. There is also a need for culture systems that permit maintenance of cells capable of differentiating into a plurality of cells types in vitro until the cells are required for subsequent cell or tissue regeneration or repair.